Balanced parabolic modulator



y 8, l952 E. J. DRAZY 2,602,919

' BALANCED PARABOLIC MODULATOR Filed Nov. 10, 1950 FIG. I

OUT

INVENTOR El J DRAZV A 7' TOQ/VE V Patented July 8, 1952 UNITED STATES PATENT OFFICE BALANCED PARABOLIC MODULATOR Elbert J. Drazy, East Orange, N. .L, assignor to Bell Telephone Laboratories,

Incorporated, I

New York, N. Y., a corporation of New York Application November 10, 1950, Serial No. 195,020

Claims. (Cl. 332-48) components of the input waves.

Oneuse to which the invention has been put is as a re eiving modulator in a measuring system. Different frequency waves are received over a line from a distant point and are used to produce difference-frequency products at the receiver. In the system referred to it was necessary to achieve a high degree of suppression of noise and other waves incoming on the line that might have frequencies lying in the band of frequencies utilized for measurement purposes, as these frequencies could not be eliminated by filtering. The modulator of the invention solved this difiiculty by producing output waves consisting to a high degree entirely of modulation products and containing substantially none of the unmodulated input wave components repeated into the output from the input. In this use the modulator was required to have broad-band characteristics and the use of input or output transformers was ruled out by practical considerations.

The obiects and features of the invention will be more fully understood from the detailed description to follow of typical circuits as illustrated in the drawings.

Fig. 1 is a schematic circuit diagram of one modulator accordin to the invention, and Fig. 2 is a similar diagram of an alternative construction according to the invention.

Waves of different frequency symbolized by the two generators ll, I2 are sent over line [0 to the receiving point where they are to be modulated against one another to produce a beat frequency to be used for measurement or signalling purposes. If one wave has a frequency f1 and the other a frequency I2, it may be desired to utilize at the receiver frequency fs=f1-fz. It is understood that other frequencies than )1 and f2 may be used and that either frequency may be variable as may be desired. It is important that no component 1: appear in the output side of the modulator except that arising from intermodulation of waves f1 and fz in the modulator. Any non-linearity in circuit elements ahead of the receiver that might produce some intermodulation would result in impressing-some freon:-

ponent on the modulator input along with; the f1 and f2 components, or especially where is represents a band of frequencies. noise in the fa range may be present on line I0. In all'such cases any impressed frequencies f: are to be prevented from reaching the output side of the r modulator.

The modulator comprises tubes [5, l6 shown for simplicity as triodes but in practice they mi ht more generally be pentodes. In the circuit of Fig. 1, the grid of tube [5 is connected through stopping condenser Hi to terminal I! of line l0,

shown terminated in resistor l3. The lower terminal I9 is connected through stopping condenser 2! to the grid of tube I6. Terminal [1 is also connected to the cathode of tube l6, while terminal I9 is also connected to the cathode of tube l5. Thus the input waves are appliedlto the grid-cathode terminals of the tubes in respectively opposite phase, or differentially, 7

Negative bias is applied to the grids from battery 2i! over bias resistors 22 and. 23 indicated as capable of separately varying the bias'voltages to the best value for each tube.

Point I9 is grounded at 24 and connected to the negative terminal of plate voltage source 25 which supplies the plates through resistors 26 and 21 respectively. The output is taken between ground at 3| and a common plate terminal 30, leading from a slider 29 on resistor 28.

The plate and grid bias voltages are adjusted to the right values to cause tube l5 to operate as a square-law modulator and tube I 6 as a grounded grid amplifier having the same gain. The input waves are repeated through tube I6 without phase reversal to the output terminals while the tube l 5 applies to the output both firstpower products reversed in phase and secondpower products.

The first-power products are made to cancel in the output, the tap 29 being adjusted to secure the best result. Such second-power products as may be applied to the output through tube l6 merely add in phase to those produced in tube I5, so that tube 16 may in fact operate on a curved part of its characteristic and still serve to cancel the unmodulated output of tube l5.

In Fig. 2, the cathodes of tubes l5 and I6 are directly connected together and through a common resistor 35 to ground. The upper terminal of the input circuit is connected through stopping condenser l8 to the grid of tube l5 while the lower terminal is connected through stopping condenser 2| to the grid of tube [6. Since normal space current flow through resistor 35 raises the cathode potential above ground, the proper grid bias is obtained from the plate battery 25 by potentiometer resistances 3G, 31 in the case of tube l5 and 38, 39 in the case of tube It.

Tube 16 operates as a grounded grid amplifier, or amplifier-modulator, driven by tube l5 acting as a cathode-follower and repeats into the common output circuit input waves in such phase as to cancel those in the output of tube l5. Tube l5 supplies both waves unchanged in frequency and modulated waves. As in thecase of Fig. 1, each tube can be made to supply both unmodulated and modulated waves into the common out put, the former being opposed to each other in phase and the latter being in like phase since they result from square-law actionin the tubes.

The circuit of Fig. 1 has a lower inputimpedance than the circuit of Fig. 2 and the lat-" ter is to be preferred where the circuit is to be driven'from a' preceding amplifier stage.

Various circuit modifications can be made without departing'irom the spirit or exceeding the scope of the invention.

What is claimed is:

'1. A modulating circuit having an input circuit for unmodulated waves and an outputcircuit for modulated waves, said input circuit and said output circuit each having one side grounded and the other side ungrounded, modu- 2. A difierential modulator comprising a pair,

of grid-controlled tubes, a circuit for impressing modulating waves of different frequency on said modulator, one terminal of said circuit connecting throughaseries condenser to the grid of one tube, the opposite terminal connecting to ground and through a series condenser to the grid of the other tube, cathode-to-ground connections for said tubes, providing a direct-current path for the space current of each tube and for causing said Waves to appear in opposite phase across the grid-cathode terminals of the respective tubes, a common anode terminal for said tubes and an output connection for the modulation products, exclusive of the input waves, connected between ground and said common anode terminal.

3. A difierential modulator according to claim 2 in which both cathodes are directly connected together and through a common impedance to ground. 4-. A differential modulator according to claim 2 including a cathode-ground impedance in the cathode-ground connection of only one tube, the other cathode being grounded, and the opposite terminalsof said resistor being connected across the terminals of said circuit.

5. A differential modulator comprising a pair or" grid-controlled tubes, an input circuitcarryin waves of 'different frequencies, opposite terminals of'said circuit connected respectively to the grids of said tubes, one terminal only of said circuit connected directly to the minus B point and through a common resistor to the cathodes of said tubes, a common terminal 'for the anodes of said tubes, and an output circuit for waves representing intcrmodulation products of said input waves without'unmodulated components thereof connected between said common terminal and the minus B point.

' 'ELBERT J. DRAZY.

REFERENCES CITED The following references are of recordin th file of this patent:

UNITED STATES PATENTS 

